Recently, optical discs are used extensively as storage media for computers and audiovisual appliances. Various formats have been proposed for optical discs so far. For example, there are read-only optical discs such as CD-ROM and DVD-ROM, write-once optical discs such as CD-R and DVD-R, and rewritable optical discs such as CD-RW, DVD-RW and DVD-RAM.
Among these various types of optical discs, CD-R, DVD-R, CD-RW and DVD-RW each include a substrate, on which lands and grooves are provided spirally, and a recording film that has been deposited on the substrate. In the optical discs of these types, data is written on tracks that are defined on the grooves.
In the write-once optical discs, the recording film thereof is made of an organic dye material. In the rewritable optical discs on the other hand, the recording film thereof is made of a phase change material. The organic dye recording film absorbs a pulsed write laser radiation, emitted from the light source of an optical disc drive, thereby causing an irreversible structural change. On the other hand, the phase-change recording film is melted when exposed to the write laser radiation, and then solidified. In this case, by adjusting the intensity and the pulse width of the write laser radiation, amorphous recording marks and crystalline spaces can be defined in the phase-change recording film. In the rewritable optical discs, the phase change of the recording film is reversible, and therefore, the recording marks are rewritable.
In any case, in writing data on an optical disc, the recording film thereof needs to be irradiated with a laser radiation that has a power that is high enough to change the structure or optical property (e.g., reflectivity, in particular) of the recording film locally. On the other hand, in reading data from an optical disc, the recording film thereof also needs to be irradiated with a laser radiation so as to detect a variation in reflectance. In this case, however, the reflectivity of the recording film should not be changed due to the exposure to the laser radiation. For that reason, the read laser radiation normally has a much smaller power than the write laser radiation.
Also, the wavelength of a read/write laser radiation changes according to the specifications to which a given optical disc conforms. For example, the read/write laser radiation for a CD has a wavelength of about 780 nm, while the read/write laser radiation for a DVD has a wavelength of about 650 nm. Generally speaking, the storage density of an optical disc depends on the beam spot size of the laser radiation. Thus, to further increase the storage density, the wavelength of the laser radiation needs to be even shorter. For that purpose, a Blu-Ray disc, which is known as a next-generation optical disc to replace DVDs in the near future, uses a violet laser radiation with a wavelength of about 405 nm.
Also, to increase the storage density and storage capacity of an optical disc, the recording film thereof needs to exhibit increased sensitivity so as to cause a structural change or phase change responsive to smaller optical energy applied. Furthermore, to perform a read or write operation on an optical disc at a higher rate, the optical disc needs to be rotated at a higher velocity. Thus, the light beam should have relatively high power not just during data writing but also during data reading.
However, as such an optical disc with an increased sensitivity is read numerous times, the recording film thereof should gradually deteriorate due to the repetitive exposure to the read laser radiation. Then, the quality of the resultant readout signal should decrease, too. Such deterioration will be referred to herein as “weak-radiation-induced deterioration”. This weak-radiation-induced deterioration is particularly significant in a recording film made of an organic dye material, but is sometimes observed in a recording film made of a phase change material, too.
Methods for avoiding such weak-radiation-induced deterioration are proposed in Japanese Laid-Open Publication No. 7-85478 and in U.S. Pat. No. 6,295,260, for example. The optical disc drive disclosed in U.S. Pat. No. 6,295,260 adaptively changes the power of the read laser radiation according to the type of an optical disc to be read. For example, in reading a CD-R in which the weak-radiation-induced deterioration occurs particularly frequently, this optical disc drive minimizes the laser radiation power.
In such an optical disc drive, however, when the power of the read laser radiation is lowered, the quality of the resultant readout signal will decrease. Also, if the laser power is decreased excessively, then the tracking error signal is detected with too low sensitivity to stabilize the tracking operation sufficiently.
In order to overcome the problems described above, preferred embodiments of the present invention provide an optical disc drive, which can perform a read operation constantly by minimizing the weak-radiation-induced deterioration of the recording film, and also provide a method for controlling such an optical disc drive.